The San Gabriel Fault is a

geological fault Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...

Los Angeles County, California Los Angeles County, officially the County of Los Angeles and sometimes abbreviated as LA County, is the List of United States counties and county equivalents, most populous county in the United States, with 9,663,345 residents estimated in 202 ...

, running about southeastward from the Ridge Basin in the

Sierra Pelona The Sierra Pelona, also known as the Sierra Pelona Ridge or the Sierra Pelona Mountains and originally known as the Liebre Mountains, is a mountain ridge in the Transverse Ranges in Southern California. Located in northwest Los Angeles County, t ...

San Emigdio Mountains The San Emigdio Mountains are a part of the Transverse Ranges in Southern California, extending from Interstate 5 at Lebec and Gorman on the east to Highway 33–166 on the west. They link the Tehachapis and Temblor Range and form the southe ...

juncture area to the western

San Gabriel Mountains The San Gabriel Mountains () are a mountain range located in Los Angeles and San Bernardino counties, California, United States. The mountain range is part of the Transverse Ranges and lies between the Los Angeles Basin and the Mojave Desert ...

that forms their southwestern face near Sunland and the northeastern

San Fernando Valley The San Fernando Valley, known locally as the Valley, is an urbanized valley in Los Angeles County, Los Angeles County, California. Situated to the north of the Los Angeles Basin, it comprises a large portion of Los Angeles, the Municipal corpo ...

, and then on the south flank to the southeastern part of the San Gabriel range.

Location and age

The San Gabriel Fault is a right-lateral strike-slip that was last active between 10 and 5 million years ago –

Late Quaternary The Holocene () is the current geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene together form the Qu ...

west of intersection with the Sierra Madre Fault, and

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS), as well as the current and most recent of the twelve periods of the ...

east of that intersection, and

Holocene The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...

only further west between Saugus and Castaic- Gorman. The San Gabriel Fault started 13-11 million years ago as a large part of the

San Andreas Fault The San Andreas Fault is a continental Fault (geology)#Strike-slip faults, right-lateral strike-slip transform fault that extends roughly through the U.S. state of California. It forms part of the tectonics, tectonic boundary between the Paci ...

, and is believed to be the former path of it. It is part of the San Gabriel Fault Zone, which starts in the northwestern corner of the Ridge Basin, splits into two faults in its southeastern section, in the northwestern

, and ends in the eastern part of those Mountains. The San Gabriel Fault is the north branch. It is older and goes eastward from the south branch which is younger, the Vazquez Creek Fault. It is also believed that the northwestern end of this fault zone meets the current San Andreas Fault zone beneath the

Frazier Mountain Frazier Mountain ( Samala: ''Toshololo'') is a broad, pine-forested peak in the Transverse Ranges System, within the Los Padres National Forest in northeastern Ventura County, California. At , Frazier Mountain is the sixteenth-highest mountain i ...

Thrust, near the juncture of the San Andreas with the

Garlock Fault The Garlock Fault is a left-lateral strike-slip fault running northeast–southwest along the north margins of the Mojave Desert of Southern California, for much of its length along the southern base of the Tehachapi Mountains. Geography Stret ...

and Big Pine Fault.

Orientation and movement

Different areas of the fault have different types of separation: normal or reverse. It is normal in the northwest portion of the fault (Castaic Hills Oil Field), and reverse/thrust in the southeast (southeast of Honor Rancho Oil Field). The San Gabriel Fault moves at a rate of between 1 and 5 millimeters a year, with an average slip of around 3 millimeters. Overall, it has 22–23 km of right separation east of the Vazquez Creek Fault, and a total amount of 42 km. Some strata dip away from the fault (in the northwestern portion) and other strata dip toward the fault (in the east, but only in the southeast portion), with the Saugus Oil Field having the greatest amount of split in the dip. The overall dip is 65°–85° NE. If these two segments were rotated horizontal, the dip becomes almost 90 degrees, and so it is thought that the dip started out to be almost vertical and rotated to its present northeast dip from folding and horizontal shortening. The fault is therefore made up of two sections. In the northwest the strike is northwest and normal separation. In the southeast there is a west-northwest strike and reverse separation. In the northern part there are mountains, the southern part has little land formations. The northern part is considered inactive, while the southeast part near Honor Rancho Oil Field is thought to be active due to evidence of Holocene sediment displacement.

References

External links

San Gabriel Fault
– USGS

– Southern California Earthquake Data Center
Geoscienceworld.org: San Gabriel Fault
{{California Faults Seismic faults of California San Gabriel Mountains Strike-slip faults Geology of Los Angeles County, California Geography of the San Fernando Valley Natural history of Los Angeles County, California